The present invention generally relates to silicone-containing materials useful as a contact lens material.
Polymeric silicone materials for contact lens applications include hydrogels and non-hydrogels. The non-hydrogels may include rigid materials, useful for rigid gas permeable contact lenses, or flexible materials useful for soft contact lenses. Whereas hydrogels typically include appreciable amounts of hydrophilic monomers and/or internal wetting agents and absorb and retain appreciable amounts of water, the non-hydrogel silicone materials do not include appreciable amounts of such hydrophilic monomers or wetting agents. Therefore, these "low water" non-hydrogel silicone materials do not absorb or retain appreciable amounts of water, for example, less than about 5 weight percent, and more typically less than about 1 or 2 weight percent.
It is important that low water silicone compositions, especially for soft contact lens applications, have desirable oxygen permeability, modulus and elastic recovery characteristics. This is particularly important in the formation of soft contact lenses, as the modulus (Young's modulus of elasticity) and elastic recovery of the lens material can have a significant impact upon lens "comfort". Lenses possessing high modulus often have a perceived stiffness; lenses with too high elastic recovery can exhibit a "suction cup" effect on the eye.
One class of silicone monomers commonly used in silicone-containing contact lens materials are bulky polysiloxanylalkyl methacrylates, e.g. methacryloxypropyl tris (trimethylsiloxy) silane (commonly referred to as "TRIS"). Since these "bulky" monomers contain a large amount of silicon per molecule, they are useful for increasing oxygen permeability of materials, a desirable property for contact lens and other biomedical device applications. However, although TRIS is known to reduce the modulus of some silicone hydrogels, i.e., polyurethane-polysiloxane hydrogel compositions, (see for example; Lai, Yu Chin, The Role of Bulky Polysiloxanylalkyl Methacrylates in Polyurethane-polysiloxane Hvdrogels, Proceedings of the American Chemical Society Division of Polymeric Materials: Science and Engineering, Vol. 72, pg. 118-119, (1995)), the use of TRIS monomeric units within "low water" silicone compositions generally increases the modulus of the resulting material. As such, TRIS monomeric units are not generally helpful in reducing the modulus of low water silicone materials.
Another class of silicone monomers known for silicone-containing contact lens materials are elastomers based on ethylenically endcapped polydimethylsiloxanes, such as the polydimethylsiloxanes disclosed in U.S. Pat. No. 4,153,641. These materials usually provide elastomers with relatively low modulus.
In summary, low water silicone materials are sought for soft contact lens applications which possess relatively low modulus, for example from 20 g/mm.sup.2 to about 150 g/mm.sup.2, and with a suitable elastic recovery rate. Furthermore, for such applications, the materials must be optically clear, manufacturable (e.g., capable of being molded or machined), have acceptable oxygen permeability, biocompatibility and resist deposit formation. Moreover, low water materials are desired that can be cast into articles such as contact lenses by methods other than free radical polymerization, for example, by injection molding.